CORROSION RESISTANCE OF THE CAST Mg ALLOYS BY NOVEL MICROSTRUCTURAL PHASE MODIFICATIONS

ABSTRACT

A method of improving the corrosion resistance of magnesium alloy castings containing more than about 2 per cent by weight of aluminum is described. The method comprises: first selecting a casting process suitable for developing at least on the surface of the casting a microstructure comprising aluminum-depleted magnesium grains surrounded by an aluminum-rich layer and preferably incorporating at least some of an intermetallic compound based on the composition Mg 17 Al 12 ; and second, heat treating at least the outer layer of the casting to promote additional intermetallic compound precipitation as required.

TECHNICAL FIELD

This application relates to a means of enhancing the corrosion resistance of cast, particularly die cast, aluminum-containing magnesium alloys through development of preferred microstructures

BACKGROUND OF THE INVENTION

Magnesium has a relative density of 1.74, the lowest of all common structural metals, and when alloyed with aluminum and zinc can exhibit strength properties generally comparable to those of aluminum alloys. This combination of low density and moderate strength makes the substitution of magnesium for more dense structural materials an attractive method for achieving significant mass reduction in automobiles.

Current magnesium applications are primarily castings due to the high productivity of casting processes (especially die casting) and the excellent castability of magnesium alloys. Magnesium is the most reactive of the common structural metals in aqueous solution, and this behavior challenges its application in automobiles which are regularly exposed to an aqueous environment commonly comprising of chloride salts. Thus, there is a continuing need to investigate and develop corrosion mitigation approaches suitable for application to magnesium alloy castings in automotive applications.

SUMMARY OF THE INVENTION

Alloys based on magnesium, and usually comprising aluminum and minor concentrations of other constituents, are widely used for the manufacture of cast articles. In the liquid state these alloys may be characterized by an average chemical composition but during the solidification process two phases frequently form—an α-phase and a β-phase. The α-phase is the major phase. It is mainly a solid solution of aluminum in magnesium that is aluminum-lean, relative to the average alloy composition. The β-phase is an intermetallic compound of magnesium and aluminum based on Mg₁₇Al₁₂ but which can exist over a range of composition and is aluminum-rich, again relative to the average alloy composition. The proportions and distributions of these phases depend on the aluminum content of the alloy and on the cooling rate which the molten alloy experiences during solidification. In some castings these β-phase particles are located proximate to the grain boundaries formed between adjacent α-phase grains and form a discontinuous array partially surrounding the α-phase regions.

The inventors have determined that modification of the proportions and distributions of these phases may be achieved by subjecting the cast articles to specific heat treatments after solidification is complete. Further, such microstructural modification may impart enhanced corrosion resistance to these heat treated magnesium-aluminum alloy based castings. This is of particular benefit in automotive applications where articles may frequently be exposed to inherently corrosive aqueous environments comprising road salt.

Specifically, a microstructure where the β-phase, rather than occurring as a series of discrete, unconnected particles is present as a continuous or near-continuous shell enclosing the α-phase, exhibits reduced corrosion rates relative to other, less desirable microstructures. Such a microstructure, when viewed in planar section, appears as a series of α-phase regions at least partially surrounded by and separated from one another by inter-connected β-phase.

Thus, in a first embodiment, it is the goal of this invention to develop a more corrosion-resistant microstructure by applying a heat treatment to a fully-solidified cast article.

In a second embodiment, the goal of this invention is to develop a more corrosion-resistant microstructure in a solidified cast article through consideration and modification of the entire production process for the article, including alloy selection and casting process. This may enable development of the desired microstructure during the casting process or enable a cast microstructure more responsive to subsequent heat treatment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the magnesium-rich region of the magnesium-aluminum phase diagram, showing equilibrium phase boundaries ab, be and ad and a schematic representation of the non-equilibrium phase boundary ax which obtains under rapid cooling.

FIG. 2A shows the microstructure of a magnesium AM50 alloy when cast and solidified at a cooling rate of 11° C./second; and FIG. 2B shows the microstructure of the same material after a 5 hour heat treatment at 232° C. The location of the α- and β-phases is indicated on each of FIGS. 2A and 2B. FIG. 2C is an enhanced contrast image of FIG. 2A and FIG. 2D is an enhanced contrast image of FIG. 2B to better show the distribution of β-phase (indicated) in the material when in the as-cast (FIG. 2A) and (as cast+heat treated) (FIG. 2B) conditions.

The micrographs are backscattered electron images obtained using a scanning electron microscope and are sensitive to composition, with heavier elements (than the magnesium matrix) displaying brighter contrast than the darker matrix.

FIG. 3 shows the average distances moved by an aluminum atom diffusing in magnesium in 100 seconds and 6 hours at temperatures ranging from 100° C. to 580° C.

FIG. 4 shows the effect of cooling rate on the volume fraction (or percentage) of the non-equilibrium eutectic β-phase formed during the casting process, and the effect of subsequent heat treatment for 6 hours at 232° C. in promoting the formation of additional β-phase for samples cooled at cooling rates of 4.3° C./second and 11° C./second. The micrographs shown in FIGS. 2A and 2B correspond to a cooling rate of 11° C./second (case (b)).

FIG. 5 shows Potentiodynamic Polarization plots for samples cooled at cooling rates of 4.3° C./second and 11° C./second in both the as cast and as cast+heat treated condition, where the heat treatment is 6 hours at 232° C. The micrographs shown in FIGS. 2A and 2B correspond to a cooling rate of 11° C./second (case (b)). The corresponding calculated uniform corrosion rates based on the corrosion current for each sample, expressed as mils (10⁻³ inches) per year or mpy are 9.6 for the as-cast microstructure (FIG. 2A) and 6.9 for the heat treated microstructure (FIG. 2B).

DESCRIPTION OF PREFERRED EMBODIMENTS

Structural magnesium alloy die castings, used for example in automobile engine cradles, are frequently exposed to corrosive environments during service, such as road splash containing salts and other contaminants. Thus, the choice of alloys for such applications is guided not only by their suitability for die casting, structural performance and cost, but also by their ability to resist corrosion in service. Ideally, a selected alloy should exhibit desired engineering traits such as strength and corrosion resistance, but frequently only premium alloys satisfy these design requirements. Hence there is benefit to improving the corrosion resistance of conventional commercial Mg—Al based alloys.

Magnesium is the most reactive structural metal in common use and hence challenges corrosion protection processes applicable to less reactive materials. For example sacrificial coatings, analogous to the zinc coatings on steel, are not readily applied to Mg and Mg alloys and while polymer- or ceramic-based coatings could provide corrosion protection, the coatings, if damaged in service, would promote more severe localized corrosion at the damage sites.

The inventors have discovered that the corrosion behavior of a specific commercial magnesium alloy (AM50), but generally extendable to all magnesium alloys containing in excess of 2% by weight of aluminum, may be significantly enhanced by supplementing the casting process with a suitable heat treatment cycle.

The invention may be best understood by referring to the magnesium-aluminum binary equilibrium phase diagram shown in FIG. 1. The behavior of commercial magnesium alloy, AM50, which has a nominal composition, by weight, of 4.40-5.40% of aluminum, and 0.20-0.60% of manganese, balance magnesium is well approximated by this diagram due to the low concentration of manganese

Inspection of FIG. 1 indicates that under equilibrium cooling conditions, that is responsive to phase boundaries indicated as lines ab and be in FIG. 1, a magnesium alloy containing 5 percent by weight of aluminum, indicated as line AA, should precipitate as a single phase α magnesium rich solid solution from the melt, becoming completely solid at about 570° C. On further cooling to below about 300° C. it will begin to precipitate β-phase, an intermetallic compound based on Mg₁₇Al₁₂ in the solid state.

It is well known that, except for special situations such as a eutectic composition, the average composition of the first-formed solid precipitated from a liquid metal alloy will differ from the composition of the liquid from which it formed leading to a phenomenon known as microsegregation which results in “coring”.

Microsegregation or “coring” during solidification in an alloy casting results from the difference in composition between the solid formed and the liquid from which it formed and the inability of the solid to maintain a uniform composition during cooling due to the relative slowness of diffusion in solids.

Thus while local equilibrium will dictate that the composition of the solid in contact with the liquid will follow the equilibrium path ab, that solid will encapsulate solid of different composition and diffusion will not occur sufficiently rapidly to render the solid homogenous. Thus the average composition of the solid under rapid cooling will be described by a line ax which lies to the magnesium-rich side of equilibrium line ab. It will be appreciated that the precise trajectory of line ax results from the alloy behavior under a particular cooling rate and that a family of lines ax will exist, each of which may be uniquely associated with a specific cooling rate.

Returning to the behavior of the Mg-5 wt % Al alloy at AA, it is clear that under non-equilibrium cooling different phases are formed during solidification from the liquid state. As solidification proceeds, the average solid composition follows the non-equilibrium solidus line ax while the liquid composition follows the composition along the liquidus line ad. Eventually the aluminum concentration in the last remaining liquid reaches the eutectic composition of about 32% Al and this last remaining liquid then solidifies to form α- and β-phase solids.

With this understanding, the non-equilibrium microstructure of FIG. 2A corresponding to an AM50 alloy casting cooled under the moderate cooling rate of about 11° C./second may be understood. The microstructure consists of aluminum-deficient α-phase grains outlined by an aluminum rich layer in which are incorporated discrete β-phase particles which may be better observed in FIG. 2C. Therefore, even at this relatively moderate cooling rate, there is inadequate time for diffusion of magnesium atoms to homogenize the α-phase solid solution.

The average distance an aluminum atom may diffuse is shown in FIG. 3 for diffusion times of 100 seconds and 6 hours respectively. Considering the average distance moved in 100 seconds, corresponding to a cooling rate of less than 10° C./second, it is clear that once the casting cools to below 300° C. essentially no additional motion of the aluminum atoms will occur during further cooling to room temperature. More significantly, even if the casting were held at the eutectic temperature of about 430° C. for a period of 100 seconds, a situation which would occur only at very slow cooling rates of less than 11° C./second, the aluminum atoms would move a distance of only about 2 micrometers. This is a significantly smaller distance than the grain size of about 40 micrometers apparent in FIG. 2A.

Thus as cooling proceeds the composition of the solid formed is deficient in aluminum and, on continued cooling, follows a trajectory such as that shown at X. As the solid is deficient in aluminum, the liquid must be enriched in aluminum and, on continued cooling, achieves the eutectic composition of about 32 wt % aluminum at the eutectic temperature of about 430° C.

In alloys of eutectic composition, the eutectic reaction generally results in a quasi-planar solidification front enabling the joint precipitation of the two eutectic phases. However, in this alloy, due to the limited volume of eutectic liquid available and due to its physical location in the constrained spaces between the α-phase grains a ‘divorced’ eutectic results. Thus substantially discrete particles of β-phase embedded in aluminum-supersaturated a forms at the boundaries between adjacent a grains. This is illustrated in FIG. 2A which is a back-scattered electron image, that is an image formed from elastically scattered electrons or electrons which have interacted with the atoms of the sample substantially without energy loss. The intensity and direction of the scattered electrons is sensitive to the atomic number of the scattering atoms, with heavy atoms being more efficient in backward scatter than lighter atoms. Hence when an image is formed from electrons scattered through an angle of approximately 180°, those regions with a high concentration of heavy elements appear brighter while those with concentrations of light elements appear dark. In AM50 alloy, magnesium (atomic number 12) will appear darker in contrast with aluminum (atomic number 13) and manganese (atomic number 25).

FIG. 2A shows magnesium grains with interiors depleted of aluminum which appear gray. The grain interiors are however outlined by a lighter-toned aluminum-rich zone in which discrete elongated particulate features corresponding to β-phase may be observed. Small light-tinted equiaxed manganese-rich particles are also observed. Thus the rejection of aluminum by the growing α-phase and the displacement of the aluminum-rich liquid to the regions between the individual α-phase grains where on continued cooling it precipitates some β-phase is shown. The distribution of β-phase may be more easily observed in FIG. 2C which is an contrast-enhanced version of FIG. 2A and shows the β-phase (and the small concentration of Manganese rich particles) as bright against the very dark α-phase grains.

However, even with β-phase precipitation, the α-phase at the grain boundary and adjacent to the β-phase is still of non-equilibrium composition and aluminum-rich. Therefore, by subjecting the cast alloy to an appropriate heat treatment additional β-phase is precipitated leading to a more continuous distribution of β-phase as shown in FIGS. 2B and 2D where FIG. 2D is a contrast-enhanced image of FIG. 2B, paralleling the relationship between FIGS. 2A and 2C. By way of example only, a suitable heat treatment practiced by the inventors entails holding the casting at a temperature of 232° C. and holding at this temperature for 6 hours; the heating rate and cooling rate are not critical. Reference to FIG. 3 reveals that the average distance moved by aluminum atoms during this heat treatment is less than a micrometer. Hence the time-temperature combination is sufficient to precipitate additional β-phase but insufficient to result in homogenization. Those skilled in the art will recognize that this particular time-temperature combination is exemplary only, and that other combinations and ranges, for example between 200° C. and 300° C. for a period of between 2 and 10 hours will enable a similar outcome. However, through understanding of the process it will be apparent that the choice of time and temperature should be informed by the need to control the average diffusion distance of the aluminum atoms. This understanding enables broad extension of the allowable heat treatment temperatures and durations such that effective short-term, high-temperature heat treatments, on the order of seconds rather than hours, may be reliably implemented.

The discussion to this point has focused on the response of castings of AM50 composition cooled at an average cooling rate of about 11° C./second. However, similar results are obtained with slower cooling rates as shown in FIG. 4. FIG. 4 indicates a progressive increase in β-phase formation with increasing cooling rate since more rapid cooling will result in a greater departure from equilibrium. In all cases however, irrespective of cooling rate, an increase in the volume fraction of 0-phase occurs with heat treatment subsequent to solidification.

The particular microstructure shown in FIG. 2B is beneficial in reducing corrosion. FIG. 5 shows potentiodynamic polarization plots for the as-cast and (as-cast+heat treated) microstructures of FIGS. 2A and 2B. Potentiodynamic polarization is an electrochemical technique by which the potential of an electrode in an electrolyte is displaced from its equilibrium potential by application of a current. The applied current is changed at a defined rate to drive the electrode potential in both anodic and cathodic directions. A procedure known as Tafel extrapolation of the polarization plots is conventionally used to assess corrosion rates.

In this case, a 1.6 wt % aqueous solution of sodium chloride (NaCl) was used as the electrolyte, and after a 60 second equilibration period at the initial scan potential, the potential was scanned over a range of ±250 mV with respect to the free corrosion potential (FCP) at a scan rate of 0.166 mV/second. Extrapolation of cathodic plots to the corrosion potential (E_(corr)) enables the corrosion current density (i_(corr)) to be read on the x-axis. The corrosion rate is directly proportional to the current flowing in the system during corrosion, and thus the corrosion current density may be used to estimate the associated uniform corrosion rate.

Because the corrosion rate of magnesium alloys varies with time and depends on the specifics of the corrosive medium, absolute values of corrosion rate are less significant than relative values. For the samples corresponding to the microstructures of FIGS. 2A and 2B, the as-cast sample (FIG. 2A) exhibits a corrosion rate about 40% greater than the (as-cast+heat-treated) sample.

It is known that addition of aluminum to magnesium leads to a higher corrosion rate initially, but with further additions of aluminum the corrosion rate either stabilizes or declines. It has been suggested that the quantity and distribution of β-phase in the alloy matrix controls this behavior. In microstructures, where the quantity of β-phase and exists as small, widely-dispersed particles, the β-phase particles act as individual cathodic sites in the anodic magnesium based matrix. This leads to a high rate of microgalvanic corrosion. However, with higher aluminum content and faster cooling rates, the fraction of β-phase will increase. Since β-phase precipitates along the grain boundaries, it eventually forms a continuous or near-continuous film adjacent to the grain boundaries. This film is presumed to act as a protective barrier layer against further corrosion, since β-phase corrodes at a lower rate than the magnesium-rich α-phase grain interior.

While such theory is not relied upon, it is clear from FIGS. 2C and 2D that heat treatment leads to the development of a more uniformly distributed and continuous β-phase in the microstructure. FIG. 4 shows the volume fractions of β-phase determined by quantitative metallography in the ‘as cast’ and ‘as cast+heat treated’ condition under two cooling rates. Case (b) with a cooling rate of 11° C./second corresponds to the case shown in FIGS. 2A-D. An increased volume fraction of β-phase was observed after heat treatment of the cast structure.

Of particular significance is that this structure, aluminum-depleted magnesium grains enveloped in a layer of β-phase, is distributed throughout the casting. Therefore, although the above mentioned theory assumes that the improved corrosion performance arises as a result of the β-phase acting as a protective barrier layer, a breach of the β-phase layer will not permanently expose the remainder of the casting to corrosive medium. Rather, a breach of the β-phase layer will initially result in rapid corrosion of the underlying aluminum depleted grains, but continued corrosion will expose another β-phase layer slowing down the progress of corrosion again. Therefore, unlike other barrier coatings, surface damage to a magnesium casting treated as described herein, will not result in continuing localized corrosion.

In practice of this invention to impart additional corrosion protection to magnesium aluminum alloys, the procedures followed should recognize the influence of interactions between alloy chemistry and casting processes.

The maximum solubility of aluminum in magnesium is about 13 weight percent at the eutectic temperature. All commercial magnesium alloys contain less than 13 weight percent aluminum. Therefore, any β-phase formation in commercial alloys is a consequence of non-equilibrium cooling. As indicated in FIG. 4 the higher the cooling rate, the higher is the fraction of β-phase in the microstructure.

Specific cooling rates depend on the particular casting process and the part geometry. For example, the production of thin-walled casings by die casting entails the use of water cooled metal molds and leads to cooling rates of about 100° C./per second or higher. By contrast, permanent mold casting of thick sections will lead to cooling rates of about 10° C./per second since the metal molds are generally preheated prior to casting and no cooling lines are used. Sand casting will typically produce cooling rates of about 1-2° C./ second.

Although different casting processes have been associated with varying ranges of cooling rates, these should be considered as illustrative and not limiting. Different regions of a cast part will be subjected to different cooling rates. For example, the first solidified region of the casting is typically subjected to a much higher cooling rate than the last formed region. Similarly a region of a die casting immediately adjacent to a cooling line will experience a higher cooling rate than a region distanced from cooling lines. Also it is known in the casting arts to selectively introduce chills and insulation into molds intended for sand casting to control and manipulate the cooling rate to modify the solidification sequence of different regions of the casting. The practice of this invention is directed to all suitable casting processes including, without limitation, die casting, permanent mold casting, sand casting (green sand and dry sand), plaster casting, investment casting as well as other processes such as lost foam casting, lost wax casting, shell mold casting and squeeze casting.

Irrespective of the casting process selected, preparation and casting of the magnesium alloy will entail the following steps. Melting the alloy and holding the molten alloy at a pouring temperature, typically between 625° C. and 725° C., which is greater than the melting temperature; introducing the molten alloy into the mold, typically by gravity except for die casting where external pressure is applied; removing the casting from the die after solidification is complete; and removing runners, sprues and risers. Generally all processes involving molten magnesium will be conducted under protective atmosphere such as about 1% sulfur hexafluoride in dry air, nitrogen or carbon dioxide.

The micrographs of FIGS. 2A/C and 2B/D clearly illustrate that additional β-phase may be precipitated from the aluminum rich shell surrounding the grain interior and the data shown in FIG. 5 illustrates the benefit in corrosion behavior resulting from this β-phase precipitation. Guidance on what constitutes an appropriate fraction of β-phase to obtain this desired result is indicated in FIG. 4 which shows that volumes of β-phase of about 3-5 percent by volume in this alloy (FIG. 4 cases (a) and (b)). This volume fraction of β-phase is effective in reducing corrosion in these samples (FIG. 5 cases (a) and (b)). Thus about 3 percent by volume of β-phase is an acceptable lower bound for the desired β-phase content to impart corrosion protection. Higher cooling rates and alloys with greater average aluminum content would develop yet larger volume fraction of β-phase, up to say 15 percent by volume, and would be expected to impart even greater corrosion protection. Thus in practice of this invention a percentage of β-phase of between 3 and 15 percent by volume is preferred.

These conceptual relationships between section size, casting method, average aluminum chemistry and volume fraction of β-phase are well known to those skilled in the art and an appropriate combination of these factors suitable for generating an appropriate fraction of β-phase may readily be determined through experience or with minimal experimentation.

Similarly, selection of an appropriate heat treatment schedule may be informed by: knowledge that successful results were achieved with a heat treatment of 6 hours at 232° C.; knowledge that the inter-diffusion coefficient of aluminum in magnesium is given by the relation:

D_(AI)(Mg)=1.2×10⁻³ exp(−143000/RT) m²s⁻¹ ; and knowledge that the average distance traveled by a diffusing aluminum atom in magnesium is given by

x =√{square root over (D _(AI(Mg)) t)}

where: R is the gas constant (=8.31447J/(mol.K))

T is the temperature in K

x is the average distance moved in meters

t is the time in seconds

Thus any suitable combination of time and temperature which will yield a value for x substantially equivalent to that obtained after 6 hours at 232° C. will be suitable and in the context of this application is considered an equivalent heat treatment.

Any heat treatment process may be selected provided no significant oxidation of the magnesium occurs. Therefore, although furnace heating may most frequently be the process of choice, other processes such as induction heating, resistance heating, radiation heating etc. may be freely selected without restriction other than suppressing oxidation.

Similarly, it is not required that the casting be heated throughout. It may be beneficial to use a process like induction hardening which will selectively heat the outer layers of the part, coupled with short heat treatment times to minimize conduction to the part interior. Thus, only the outer layers of the cast part would be processed to enhance corrosion resistance. This might have benefit in combination with an overall heat treatment intended to improve the mechanical properties of the casting.

For example, consider a precipitation-hardening, aluminum-containing magnesium alloy such as AZ91 (major components by weight: aluminum, 8.3-9.7%; zinc, 0.35-1.0%; balance magnesium). An AZ91 casting could be subjected to a first heat treatment to develop a precipitation hardened microstructure then subjected to a brief induction hardening cycle, possibly followed by a quench, to develop enhanced corrosion resistance extending only from the surface to a selected depth in the casting.

The preceding may best be understood by consideration of two examples, representative of two embodiments of the invention.

EXAMPLE 1

It is often desired to impart enhanced corrosion protection to a cast article where the casting process and magnesium alloy have been selected and the article may be in production. In this case only moderate process changes are desired. These changes could include restricting the alloy composition within the alloy specification by accepting only alloys with the richest aluminum content consistent with the specification. Alternatively or additionally, it may be feasible to effect minor changes to the casting cooling rate, for example by increasing coolant flow in a die casting mold or possibly by introducing a chill into a sand casting. In this case, the corrosion resistance can be enhanced by subjecting the casting to the previously-described heat treatment to precipitate β-phase or additional β-phase to promote a more continuous distribution of β-phase.

As a necessary precursor, the combination of the alloy composition and the casting process must render an inhomogeneous and non-equilibrium microstructure similar to that shown in FIG. 2A—that is, grains of aluminum-depleted α-phase outlined by a layer of at least aluminum enriched α-phase, or, more preferably, aluminum enriched α-phase adjoining β-phase particles. This requirement must at least be satisfied on those surfaces of the casting which will contact the corrosive medium.

Subsequent heat treatment will be based on subjecting the casting to a heat treatment comprising holding the casting at 232° C. for 6 hours or its functional equivalent calculated based on diffusion considerations as previously described.

The desired outcome is to develop a microstructure comprising a continuous or near-continuous distribution of β-phase.

EXAMPLE 2

When a part of original design is considered for casting, or where adjustment to the composition or casting process of an existing casting is feasible the following process should be followed:

-   -   (a) make an initial selection of a casting process and suitable         alloy composition based on the need to provide desired         attributes such as strength and surface finish to the casting         while subject to constraints such as piece cost and production         rate;     -   (b) determine whether the casting will be exposed to a corrosive         atmosphere. If not, terminate the selection procedure and         finalize the alloy composition and casting process under         consideration, otherwise continue to step (c);     -   (c) determine the cast microstructure by consideration of the         alloy composition, particularly its aluminum content and the         cooling rate of the casting process. This may be based on         experience, experiment or modeling; then         -   i. if the anticipated microstructure has the characteristics             suitable for the required level of corrosion protection,             finalize the alloy composition and casting process under             consideration, otherwise continue to step (ii);         -   ii. If the anticipated microstructure is generally suitable             but does not provide the desired level of corrosion             protection but may be rendered suitable through heat             treatment, add and incorporate the heat treatment into the             process and finalize the alloy composition and casting             process under consideration. Otherwise continue to step             (iii);.         -   iii. If the anticipated microstructure is generally suitable             but does not provide the desired level of corrosion             protection and cannot be rendered suitable by heat treatment             or if the microstructure is generally unsuitable, then             return to step (a), select a different composition and/or             casting process and repeat steps (b) and (c).

Thus, in summary, the inventors have discovered a means of reducing the corrosion rate of aluminum-containing magnesium castings by adjusting the aluminum content of the alloy in congruence with the casting process and therefore the cooling rate imposed during solidification. Further the inventors have determined that additional improvements in corrosion resistance may be imparted to the casting through application of a wide range of heat treatment methods.

While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements. 

1. A method of improving the corrosion behavior of magnesium alloy castings comprising an average composition of more than about 2 per cent by weight of aluminum, by developing a microstructure in which regions of varying aluminum content are functionally arranged to impart improved corrosion resistance, the method comprising: cooling the casting at a rate sufficient to develop in the casting a microstructure comprising regions of less than average aluminum content generally surrounded by a layer of at least greater than average aluminum concentration; and subjecting the casting to a heat treatment at a temperature and for a duration sufficient to promote an altered microstructure comprising an at least partially continuous network of a magnesium-containing intermetallic compound which at least partially encloses the regions of less than average aluminum concentration.
 2. The method of claim 1 wherein the intermetallic compound is based on Mg₁₇Al₁₂.
 3. The method of claim 1 wherein the heat treatment comprises holding at a temperature of about 232° C. for a period of about 6 hours.
 4. The method of claim 1 wherein the heat treatment comprises holding at temperature of between 200° C. and 300° C. for a period of between 2 hours and 10 hours.
 5. The method of claim 1 wherein the casting is manufactured by a casting process is selected from the group consisting of die casting, permanent mold casting, green sand casting, dry sand casting and investment casting.
 6. The method of claim 1 wherein the aluminum content of the alloy is in the range of about 3 percent by weight to about 10 percent by weight.
 7. The method of claim 1 wherein the heat treatment process is conducted using one of the group consisting of furnace heating, induction heating and radiant heating.
 8. The method of claim 1 wherein the corrosion performance of the alloy is determined by the technique of Potentiodynamic polarization.
 9. A method of imparting a predetermined level of corrosion resistance to magnesium alloy castings comprising aluminum as a major alloying element, by development of a microstructure comprising an at least partially continuous network of a magnesium-containing intermetallic compound which at least partially encloses regions of a magnesium-rich solid solution, the method comprising: (a) selecting a casting process and alloy composition; (b) determining, based on experience, experiment or modeling, the cast microstructure by consideration of the alloy composition, particularly its aluminum content, and the cooling rate of the casting process; then (c) assessing, based on experience, experiment or modeling the level of corrosion protection conveyed by the microstructure, and; either (d) terminating the method or enhancing the corrosion protection by; either (e) heat treating the casting; or (f) by selecting a different alloy composition or casting process and repeating the above steps beginning with step (b).
 10. The method of claim 9 wherein heat treating the casting comprises subjecting the casting to a heat treatment of suitable duration and at suitable temperature to promote an at least partially continuous distribution of a magnesium-containing intermetallic compound.
 11. The method of claim 9 wherein the heat treatment comprises holding at a temperature of about 232° C. for a period of about 6 hours.
 12. The method of claim 9 wherein the heat treatment comprises holding at a temperature from between 200° C. and 300° C. for a period of between 2 hours and 10 hours.
 13. The method of claim 9 wherein the casting is manufactured by a casting process is selected from the group consisting of die casting, permanent mold casting, green sand casting, dry sand casting and investment casting.
 14. The method of claim 9 wherein the aluminum content of the alloy is in the range of about 3 percent by weight to about 10 percent by weight.
 15. The method of claim 9 wherein the corrosion resistance is assessed using Potentiodynamic Polarization.
 16. An aluminum-containing magnesium alloy article of specified average aluminum concentration heat treated to develop a microstructure comprising an at least partially continuous network of a magnesium-containing intermetallic compound which at least partially encloses regions of a magnesium-rich solid solution and thereby impart enhanced corrosion protection to the article.
 17. The corrosion-resistant, aluminum-containing magnesium alloy article of claim 14 wherein the magnesium-containing intermetallic compound is based on the composition Mg₁₇Al₁₂.
 18. The corrosion-resistant, aluminum-containing magnesium alloy article of claim 15 wherein the aluminum-magnesium intermetallic compound based on the composition Mg₁₇Al₁₂ is present in an amount ranging from 3 to 15 per cent by volume. 